This present invention relates to an antenna element and a portable information terminal, particularly to an antenna element used in a portable telephone and a portable telephone using the antenna.
As an antenna element for transmission and reception in portable telephones, there has been hitherto known, for example, a monopole antenna or a helical antenna set up to extend in the longitudinal direction of their case.
The impedance of such an antenna element is different from the impedance of a radio section inside the portable telephones. It is therefore necessary to match these impedances with each other. For this purpose, in conventional portable telephones, a matching circuit is arranged between their radio section and their antenna element.
In recent years, a portable information terminal which can achieve two functions as a portable telephone and a personal handy-phone system (PHS) has been developed. A portable telephone and a PHS are different from each other in the radio frequencies (band) used for transmission and reception of information. In the case that information communication is carried out in any one band, an antenna is generally designed in the manner that the voltage standing wave ratio (VSWR) of the antenna is set to 2 or less in the band. It is therefore necessary that in the portable information terminal which can achieve two functions as a portable telephone and a PHS, the VSWR of its antenna is set to 2 or less in plural bands or a broad band. However, in conventional antennas having such a matching circuit as described above, the band in which the VSWR of the antennas is 2 or less is narrow. Thus, it is difficult that the conventional antennas are used as portable information terminals having the above-mentioned plural functions.
The conventional matching circuit is composed of lumped-parameter elements such as a coil and a condenser. Therefore, when electrical signals are transmitted from a radio section to an antenna element through the matching circuit, loss is generated in the coil and condenser in the matching circuit to cause a problem that transmission efficiency of the electrical signals is lowered.
Thus, the present invention has been made to overcome the above-mentioned problems.
An object of the present invention is to provide an antenna element and a portable information terminal in which loss of electrical signals is small to give high efficiency.
Another object of the present invention is to provide an antenna element and a portable information terminal having a broad usable band.
The antenna element according to the present invention includes a first antenna part which is substantially equivalent to a series resonant circuit, and a second antenna part which is brought with contact with and connected to the first antenna part and is substantially equivalent to a parallel resonant circuit.
In the antenna element having this structure, the first antenna part is substantially equivalent to the series resonant circuit and the second antenna part is substantially equivalent to the parallel resonant circuit. Therefore, the first antenna part and the second antenna part have impedance characteristics reverse to each other. By jointing the two antenna parts having the reverse impedance characteristics in above-mentioned way, their reactances are mutually cancelled out. In this way, the impedance of the antenna element and that of a radio section can be matched with each other so that a usable band can be made broad without using any matching circuit.
Since the impedances can be matched with each other by jointing the two antenna parts, it is unnecessary to set a matching circuit as seen in the prior art. As a result, loss of electrical signals in the matching circuit can be prevented. Thus, the antenna element of the present invention has a high efficiency.
Preferably, the first antenna part and the second antenna part are fitted in series to a feeding point.
Preferably, the first antenna part and the second antenna part are fitted in parallel to a feeding point.
Preferably, the first antenna part includes a plate antenna, and the second antenna part includes a linear antenna.
Preferably, the linear antenna includes at least one selected from the group consisting of a monopole antenna and a helical antenna.
Preferably, the antenna element further includes a substrate whose surface has an electrical conductivity; the first antenna part is set up, through a dielectric, over the surface of the substrate; and the second antenna part is set up to extend from the substrate.
In this case, the first antenna part is set up, through the dielectric, over the surface of the substrate; therefore, the wavelength of a radio wave advancing in the first antenna part can be made short. As a result, the length of the first antenna part can be made short so that the size of the antenna element can be made small. The second antenna part is set up to extend from the substrate; therefore, the second antenna part can transmit and receive the radio wave certainly without being affected by the substrate.
Preferably, the antenna element further includes a substrate whose surface has an electrical conductivity; and the first antenna part and the second antenna part are set up, through a dielectric, over the surface of the substrate. In this case, the first antenna part and the second antenna part are set up, through the dielectric, over the surface of the substrate; therefore, the wavelength of a radio wave advancing in the first and second antenna parts can be made short. As a result, the size of the first and second antenna parts can be made small so that the size of the antenna element can be made small.
Preferably, the second antenna part includes at least one selected from the group consisting of a monopole antenna, a helical antenna, a meander line antenna, and a zigzag antenna.
The portable information terminal according to the present invention includes an antenna element including a first antenna part which is substantially equivalent to a series resonant circuit, and a second antenna part which is brought with contact with and connected to the first antenna part and is substantially equivalent to a parallel resonant circuit.
In the portable information terminal having this structure, the first antenna part is substantially equivalent to the series resonant circuit and the second antenna part is substantially equivalent to the parallel resonant circuit. Therefore, the first antenna part and the second antenna part have impedance characteristics reverse to each other. By jointing the two antenna parts having the reverse impedance characteristics, their reactances are mutually cancelled out. Thus, the impedance of the antenna element and that of a radio section can be matched with each other. As a result, the portable information terminal has a broad usable band.
Since the impedances can be matched without using a matching circuit as seen in the prior art, loss of electrical signals in the matching circuit is not generated. Thus, the portable information terminal of the present invention has a high efficiency.